Methods of manufacturing casted articles, and systems

ABSTRACT

A method for manufacturing a casted article is presented. The method includes steps of forming a casted article by a liquid metal cooled directional solidification process, removing a metallic material from a surface of the casted article and inspecting the surface of the casted article. The surface of the casted article is inspected for the presence of the metallic material by exposing the surface to a visualization reagent. A system for manufacturing the casted article is also presented.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. patent applicationSer. No. 12/334582, entitled “A PROCESS FOR REMOVING METALLIC MATERIALFROM CASTED SUBSTRATES AND RELATED COMPOSITIONS,” filed on Dec. 15,2008, which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates generally to casted articles, and morespecifically to cleaning the casted articles, for example, turbineengine components.

Liquid metal cooling is often used to form high-gradient castings ofsuperalloy components in advanced gas turbines, as well as otherindustrial parts. During the cooling process, some of the molten metalused to cool the casting can breach the casting container and bedeposited as a contaminant on the surface of the casted article. Thecasted article is typically subjected to a series of thermal fabricationand heat treatment cycles before becoming a useful casted article. Themetal contaminant, if present, can diffuse below the surface of thecasted item during the thermal processing cycles, and precipitouslyaffect the surface quality and bulk properties of the finished article.

Accordingly, there is a need for methods of effectively removing suchmetallic contaminants. It would also be desirable if the processes didnot result in the formation of an unacceptable amount of hazardousfumes. The processes should also exhibit some degree of selectivity. Forexample, the process should effectively remove the metallic contaminantwhile substantially preserving the casted article.

BRIEF DESCRIPTION

Embodiments of the invention are directed towards a method formanufacturing a casted article.

According to one embodiment of the invention, a method for manufacturinga casted article is disclosed. The method includes steps of forming acasted article by a liquid metal cooled directional solidificationprocess, removing a metallic material from a surface of the castedarticle and inspecting the surface of the casted article. The surface ofthe casted article is inspected for the presence of the metallicmaterial by exposing the surface to a visualization reagent.

In another embodiment, a method comprises filing a mold with a moltenmetal and immersing the mold into a cooling metallic liquidprogressively to cause a solidification interface to pass through themolten metal. The method further includes recovering a casted articlefrom the mold and removing a metallic material from a surface of thecasted article. Furthermore, the method includes inspecting for thepresence of the metallic material on the surface of the casted articleby exposing the surface to a visualization reagent.

Another embodiment of the invention is a system, comprising a heatingfurnace at a temperature above the liquidus temperature of a metalwithin a mold, a liquid cooling bath comprising a cooling metallicliquid and a bath having an aqueous composition. The aqueous compositionremoves a metallic material from a surface of a casted article. Thesystem further includes a unit to expose at least a portion of thecasted article to a visualization reagent, and a visualization aid forobserving a response of the visualization reagent.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a flow diagram depicting a method of making a casted articlein accordance with an embodiment of the invention.

FIG. 2 is a schematic view of a system (furnace) for casting process inaccordance with an embodiment of the invention.

FIG. 3 is a schematic view of a cleaning bath as recited in the methodshown in FIG. 1, in accordance with an embodiment of the invention.

FIG. 4 is a flow chart depicting a method of inspecting (testing) asurface of a casted article in accordance with an embodiment of theinvention.

FIG. 5 is a schematic view of the inspection method as recited in FIG.3, in accordance with an embodiment of the invention

DETAILED DESCRIPTION

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about,” is not limited to the precise valuespecified. In some instances, the approximating language may correspondto the precision of an instrument for measuring the value.

In the following specification and the claims that follow, the singularforms “a”, “an” and “the” include plural referents unless the contextclearly dictates otherwise.

As used herein, the term “superalloy” refers to a nickel, cobalt oriron-based heat resistant alloy that has superior strength and oxidationresistance at high temperatures. Nickel and cobalt-based alloys arefavored for high-performance applications. The superalloy can containchromium to impart surface stability and one or more minor constituentssuch as molybdenum, tungsten, titanium, iron or aluminum forstrengthening purposes. Physical properties of the superalloy make thesuperalloy particularly useful for manufacturing a gas turbinecomponent.

During a representative casting process, a casted article getscontaminated by the deposition of metal contaminants on the surface ofthe casted article. Various methods or techniques can be used to stripthe deposited metal contaminant from the surface of the casted article.Even after effective cleaning or striping processes, there is apossibility of some residual metal contaminants to be present on thesurface. As fully cleaned articles or components are desired, it isnecessary to determine if the surface of the article is cleaned or stillcontains the metal contaminants. To ensure quality of the surface of thecasted article, an inspection test of the surface is performed to detectthe presence of any metal contaminants on the surface, according to anembodiment of the present invention. A visualization reagent can be usedfor the detection of the residual metal present on the surface. Theresponse of the visualization reagent indicates the presence or absenceof any metal contaminant on the surface. The testing of the surface, inthis way, further assures the effectiveness of the cleaning processused.

As used herein, the term “visualization reagent” refers to a chemicalreagent that gives an optical response on reacting with a metallicmaterial. The optical response of the chemical reagent can typically beobserved in the visible region of the electromagnetic radiation.

According to an embodiment of the invention, a method of making a castedarticle is described with the help of a flow diagram 10 as illustratedin FIG. 1. The method involves steps 12 and 14 of manufacturing andcleaning of the casted article, respectively. The manufacturing method12 includes pouring a molten metal into a mold as indicated by step 16and solidifying the molten metal directionally through a liquid coolingmethod as indicated by step 18. Step 20 of the manufacturing methodinvolves removing the mold from a cooled liquid metal. At step 22, acasted article is recovered from the mold. The method further involvesstep 14 for cleaning of the casted article. The cleaning of acontaminated surface includes steps of removing the metallic material orthe metal contaminant from the surface as indicated by step 24 followedby inspecting the surface for the presence of any residual metallicmaterial as indicated by step 26.

Referring now to FIG. 2, a directional solidification furnace 40 isillustrated, in one embodiment, for manufacturing a casted article bythe method 10 as described in FIG. 1. The directional solidificationfurnace 40 includes a heating furnace 42 and a liquid metal cooling bath54. The heating furnace 42 is heated, for example, by resistive heatingusing graphite strips 44 within an insulated furnace box 46. A ceramicmold 48 is located within the furnace box 46 by a mold positioner 50.The mold 48 is filled with a molten metal 49 to be casted. The heatingfurnace 42 is heated to a sufficiently high temperature to ensure thatthe metal 49 in the mold 48 is melted (that is to a temperature abovethe liquidus temperature of the metal 49). Directional solidification isachieved by lowering the mold 48 containing the molten metal 49 out ofthe heated furnace box 46 into the liquid metal cooling bath 54 throughan aperture 52 in the furnace box 46 and solidification progresses frombottom to top within the mold 48. The heating furnace 42 heats the metalalloy and the liquid cooling bath 54 removes heat from the metal alloyduring a casting process and forms a casted article. The liquid coolantbath 54 includes a cooling metallic liquid contained in a crucible 56 ofa metal or a refractory material.

The casted article may include a metal, or a metal alloy. In oneembodiment, the casted article includes iron, cobalt, nickel, aluminum,chromium, titanium, and mixtures or alloys, for example, stainlesssteel. In a preferred embodiment, the casted article includes asuperalloy.

The mold 48 is chosen according to the desired shape of the castedarticle and may differ from the shape as shown in FIG. 1. The castedarticle can be of many shapes. As described herein, in one embodiment,the casted article is a component of a turbine engine, for example, anairfoil, a blade or “bucket.” While the molded article is discussed incertain embodiments as a component for a turbine engine, the mold andin-turn, the molded article, can be in any shape including bothsymmetric and irregular shapes.

The liquid metal cooling bath 54, in the illustrated embodiment of FIG.2, includes a cooling metal with a melting points less than about 700degree Celsius. Suitable cooling metals may include for example,aluminum, tin, lithium, magnesium, zinc, gallium, indium or acombination thereof. In a preferred embodiment, the liquid metal coolingbath may include tin as a cooling metal to remove heat from the castingmetal alloy. During cooling, metals from the bath may penetrate the mold48 and contaminate the casted article.

According to an embodiment of the invention, a metallic material or ametal contaminant is deposited on a surface of a casted article as aresult of the ingression of liquid metal during a casting process. Theingression of liquid metal may occur when a mold containing the castedarticle cracks while the mold is still immersed in a liquid metalcooling bath. When the mold cools and develops cracks while still in theliquid metal cooling bath, the liquid metal can flow along the cracks inthe mold and eventually make contact with the surface of the castedarticle inside the mold. The liquid metal can react with the interior ofthe mold while flowing through the mold cracks and with the articlematerial while in contact with surfaces of the casted article. For thisreason, some elements in the mold and in the casted article may also bepresent in the metal contaminant due to the interaction of the ingressedliquid metal with the mold and casted article materials.

As used herein, the “metallic material” is a material containing metalor metal alloys. The metallic material is deposited on the surface ofthe casted article in excess of any amount of the material which amountmay be present in the casted article. In a non-limiting example, themetallic materials may include at least one element selected from thegroup consisting of tin, iron, cobalt, nickel, aluminum, chromium,titanium, and combinations thereof, for example stainless steel. Themetallic material may further include other modifying constituentsco-deposited with the metal or metal alloy, such as silicon, zirconium,yttrium, oxygen or combinations thereof.

The thickness of the metallic material deposited on the surface willdepend on various factors, such as the type of the article being cast,the casting process employed, the materials being employed, and thelike. In one embodiment, the metallic material may have a thicknessbetween about 2 microns and about 2000 microns. In another embodiment,the metallic material may have a thickness between about 5 microns andabout 1000 microns. In yet another embodiment, the metallic material mayhave a thickness between about 10 microns and about 500 microns.

The cleaning method 14 of the casted article, in the illustratedembodiment of FIG. 1, typically includes a chemical process for removingthe metallic material from the surface. The chemical process includescontacting (treating) the contaminated surface containing the metallicmaterial with an aqueous composition. Various techniques can be used totreat the casted article with the aqueous composition. For example, thecasted article can be continuously sprayed with the composition, usingvarious types of spray guns. A single or multiple spray guns could beemployed. In another alternative embodiment, the aqueous compositioncould be poured over the casted article and continuously recirculated.

The cleaning method 14 of the article has a very desirable degree ofselectivity. In other words, the metallic material can be effectivelyremoved from the surface of the casted article, without adverselyaffecting or damaging the article. This is very advantageous forpreserving the structural integrity and dimensions of the castedarticle. Moreover, the treatment composition described herein isrelatively benign, from an environmental standpoint, as compared tomineral acid-based compositions.

As used herein, the term “removing the metallic material” is meant torefer to the severe degradation of the metallic material, leaving atmost only a metallic material residue. The residue weakly adheres to theunderlying surface. The residue can be removed by a subsequent,conventional technique such as “de-smutting,” as discussed below.

In an embodiment, FIG. 3 illustrates a system 60 of treating a castedarticle 62 recovered from the directional solidification furnace 40 asillustrated in FIG. 1, with an aqueous composition 68. The system 60includes a bath 70 filled with the aqueous composition 68. The castedarticle 62 includes portions containing a metallic material 66 presenton a surface 64. The casted article 62 is immersed into the aqueouscomposition 68 to strip the metallic material 64 from the surface 62.Immersion in this manner, often permits the greatest degree of contactbetween the aqueous composition 68 and the metallic material 64 beingremoved. A treated article 72 is removed from the bath 70 after apredetermined time.

Immersion time and bath temperature will depend on various factors suchas the type of metallic material being removed, the aqueous compositionbeing used in the bath, and the equipment capabilities. In oneembodiment, the treated article 72 is free of any residual metallicmaterial. In another embodiment, the treated article 72 contains theresidual metallic material 76 on a surface 74.

Usually, the bath 70 filled with the aqueous composition 68 ismaintained at a temperature in the range of about room temperature toabout 100 degrees Celsius, while the article 49 is immersed therein. Inpreferred embodiments, the temperature is maintained in the range ofabout 45 degrees Celsius to about 95 degrees Celsius. The immersion timein the bath may vary considerably. It is usually in the range of about10 minutes to about 72 hours, and preferably, from about 1 hour to about20 hours. Longer immersion times may compensate for lower bathtemperature.

In one embodiment, the aqueous composition 68 or the treatmentcomposition includes an acid having the formula H_(x)AF₆. In thisformula, A is selected from the group consisting of Si, Ge, Ti, Zr, Al,and Ga. The subscript x is a quantity from 1 to 6, and more typically,from 1 to 3. Materials of this type are available commercially, or canbe prepared without undue effort. The H_(x)AF₆ compound, sometimesreferred to herein as the “primary acid”, is preferably H₂SiF₆ orH₂ZrF₆, or mixtures thereof. In some embodiments, H₂SiF₆ is especiallypreferred. The compound H₂SiF₆ is referred to by several names, such as“fluosilicic acid,” “hydrofluosilicic acid,” “fluorosilicic acid,” and“hexafluorosilicic acid.”

Precursors to the H_(x)AF₆ acid may also be used. As used herein, a“precursor” refers to any compound or group of compounds that can becombined to form the acid or its dianion AF₆ ⁻², or which can betransformed into the acid or its dianion under reactive conditions, forexample, the action of heat, agitation, catalysts, and the like. Thus,the acid can be formed in situ in a reaction vessel, for example.

As one illustration, the precursor may be a metal salt, inorganic salt,or an organic salt in which the dianion is ionically bound. Non-limitingexamples include salts of Ag, Na, Ni, K, and NH₄ ⁺, as well as organicsalts, such as a quaternary ammonium salt. Dissociation of the salts inan aqueous solution yields the acid. In the case of H₂SiF₆, a convenientsalt, which can be employed, is Na₂SiF₆.

In one embodiment, H₂SiF₆ can be formed in situ, for example, by thereaction of a silicon-containing compound with a fluorine-containingcompound. An exemplary silicon-containing compound is SiO₂, while anexemplary fluorine-containing compound is hydrofluoric acid, that isaqueous hydrogen fluoride.

When used as a single acid, the H_(x)AF₆ acid can be somewhat effectivefor removing chromide deposition. The preferred level of acid employedwill depend on various factors, such as the type and amount ofdeposition being removed; the location of the metallic material on thearticle; the type of the article; the thermal history of the article anddeposition, for example, the level of interdiffusion; the technique bywhich the article is being exposed to the treatment composition asdescribed above; the time and temperature used for treatment; and thestability of the acid in solution.

In general, the H_(x)AF₆ acid is present in the treatment composition ata level in the range of about 0.05 M to about 5 M, where M representsmolarity. Molarity can be readily translated into weight or volumepercentages, for ease in preparing the solutions. Usually, the level isin the range of about 0.2 M to about 3.5 M. In the case of H₂SiF₆, apreferred concentration range is often in the range of about 0.2 M toabout 2.2 M. Longer treatment times and/or higher treatmenttemperatures, described below, may compensate for lower levels of theacid, and vice versa. Adjustment of the amount of H_(x)AF₆ acid, and ofother components described below, can readily be made by observing theeffect of particular compositions on the deposition removal from thearticle.

In preferred embodiments, the treatment composition also includes atleast one additional acid or “second acid” or precursor thereof. Theadditional or “second” acid is preferably a phosphorous-containingcompound, or nitric acid. Non-limiting examples of the phosphorouscompounds include phosphoric acid and phosphorous acid, as well asmixtures thereof. In general, the phosphorous compounds are commerciallyavailable, as is nitric acid. These compounds can also be synthesized bywell-known techniques.

Those skilled in the art can select the most appropriate additionalacid, based on observed effectiveness and other factors, such asavailability, compatibility with the primary acid, cost, andenvironmental considerations. Moreover, a precursor of the acid may beused, such as a salt, as described above in reference to the primaryacid. For most embodiments, the preferred additional acid is aphosphorous compound, with phosphoric acid being especially preferred.

The present inventors do not wish to be bound to any particular theoryin regard to the unexpected efficacy of the phosphorous compounds andnitric acid. However, they appear to provide the acidic capacity torapidly oxidize the metal in the metallic material. This in turn appearsto induce the metallic material to become solubilized, and to readilydetach from the casted article surface region.

The amount of additional acid employed, that is, the phosphorouscompound or nitric acid, will depend on the acid itself, as well as theidentity of the primary acid, and on many of the factors set forthabove. Phosphorous compounds are usually present in the composition at alevel in the range of about 0.1 M to about 20 M. In some preferredembodiments, for example, in the case of phosphoric acid, the preferredrange is from about 0.5 M to about 5 M. Furthermore, some preferredembodiments contemplate a range of about 2 M to about 4 M.

When present as the additional acid, nitric acid is present at a level,which will minimize degradation of casted articles being treatedaccording to this invention. Usually that level will be no greater thanabout 1.2 M. In preferred embodiments, the range will be from about 0.3M to about 1 M.

In some embodiments, the treatment composition 68 includes a minoramount of a third acid. This constituent is usually a strong acid,having a pH of less than about 3.5 in pure water. Thus, the third acidcan be nitric acid, when the second acid is a phosphorous compound.Non-limiting examples of other strong mineral acids are sulfuric acid,hydrochloric acid, hydrofluoric acid, hydrobromic acid, hydriodic acid,perchloric acid, alkyl sulfonic acids, and mixtures of any of theforegoing. The strong acid appears to be especially useful for removingportions of the metallic material, which metallic material may havediffused into the casted article.

In one embodiment, the third acid comprises hydrochloric acid, nitricacid, or mixtures thereof. In a preferred embodiment, the third acid ishydrochloric acid. Typically, the acid is advantageously supplied andused in aqueous form, for example, 35-38 percent hydrochloric acid inwater.

The amount of third acid employed will depend on the identity of theprimary acid and the second acid, and on many of the factors set forthabove. To minimize degradation of the article, the third acid ispreferably present at the levels described above, in regard to nitricacid. Thus, the concentration of the acid in the treatment compositionis usually no greater than about 1.2 M, and preferably in the range ofabout 0.3 M to about 1 M.

The aqueous composition 68 of the present invention may further includevarious other additives, which additives serve a variety of functions.Non-limiting examples of these additives are inhibitors, dispersants,surfactants, chelating agents, wetting agents, deflocculants,stabilizers, anti-settling agents, reducing agents, and anti-foamagents. Those of ordinary skill in the art are familiar with specifictypes of such additives, and effective levels of usage. An example of aninhibitor for the composition is a relatively weak acid like aceticacid. Such a material tends to lower the activity of the primary acid inthe composition. This is desirable in some instances, for example, todecrease the potential for pitting of the surfaces of some types ofcasted articles, if contacted with the treatment composition.

Treatment of the casted article 62 in the aqueous composition 68severely degrades the integrity of the metallic material 66 beingremoved. The degraded metallic material is referred to herein as “smut”or “metallic material residue.” The metallic material residue oftencontinues to weakly adhere to an underlying layer of the casted article.Consequently, the treatment is usually followed by a post-strippingstep, often referred to as a “de-smutting” operation. Such a step isknown in the art, and described in various references. De-smutting maybe performed in the form of a gentle abrasion step that minimizes damageto the casted article or the underlying layer. As one example,grit-blasting can be carried out by directing a pressurized air streamcontaining aluminum oxide particles across the article surface. The airpressure is usually less than about 100 psi. The grit-blasting iscarried out for a time period sufficient to remove the degradeddeposition. The duration of grit-blasting in this embodiment will dependon various factors, such as the thickness and specific composition ofthe smut layer; the size and type of grit media, and the like. Theprocess is typically carried out for about 30 seconds to about 3minutes.

Other known techniques for abrading the surface may be used in lieu ofgrit-blasting. For example, the article surface can be manually scrubbedwith a fiber pad, such as a pad with polymeric, metallic, or ceramicfibers. Alternatively, the article surface can be polished, for example,with a flexible wheel or belt in which alumina or silicon carbideparticles have been embedded. Liquid abrasive materials mayalternatively be used on the wheels or belts. These alternativetechniques should be controlled in a manner that maintains a contactforce against the article surface that is no greater than the force usedin the grit-blasting technique discussed above.

Other techniques, or combinations of techniques, can be employed inplace of abrasion, to remove the degraded metallic material. Examplesinclude laser ablation of the article surface, or tumbling of the castedarticle, including water tumbling. Alternatively, the degraded materialcould be scraped off the article surface. As still another alternative,sound waves, for example, ultrasonic waves, could be directed againstthe surface, causing vibrations to shake loose the degraded material.For each of these alternative techniques, those skilled in the art wouldbe familiar with operating adjustments which are made to control therelevant force applied against the surface of the casted article, as inthe case of the abrasion technique, to minimize damage to the articlebeing preserved. The article is sometimes rinsed after this step, forexample, using water or a combination of water and a wetting agent.

The treated casted article 72, in the illustrated embodiment of FIG. 3,may still contain some portion of residual metallic material 76 on thesurface 74. The presence of the metallic material 76 on the surface 74means that the surface 74 is still contaminated even after the cleaningtreatment and may require additional processing.

According to one embodiment of the invention, the cleaning method 14further includes a method 26 of inspecting a cleaned casted article, asillustrated in FIG. 1. The method 26 uses a visualization reagent toassist in the detection of the presence of a metallic material on asurface by observing response of the reagent. The method involves thesteps of treating the surface of the casted article with an extractionreagent, exposing the treated surface to the visualization reagent andobserving a response of the visualization reagent. Reaction of themetallic material with the extraction reagent forms a compound, whichresponds when brought in contact with the visualization reagent.

Suitable techniques to treat the surface with the extraction reagent mayinclude swiping the surface with the extraction reagent, pouring theextraction reagent over the surface or the like. In one embodiment, theextraction reagent includes an acid. Suitable acids as the extractionreagent may include, but not limited to, hydrochloric acid, nitric acid,sulfuric acid, phosphoric acid, acetic acid or a combination thereof.

A variety of visualization reagents can be used for the detection of themetallic material. Suitable visualization reagents may include, but notlimited to, a calorimetric reagent or a chemical indicator, afluorescent dye or an optically active material. The “optically activematerial” as used herein may refer to a material, which absorb, scatteror deflect light. The response is observed on exposure of the reagent tothe surface of the casted article. The visualization reagent may resultin a calorimetric response that is a change in color, absorbance,scatter or fluorescence response corresponding to the visualizationreagent used.

In a preferred embodiment, a calorimetric reagent is used for thedetection of the metallic material on the surface. Change in color maydepend on the type of the metallic material present on the surface andthe calorimetric reagent used for the detection. Various calorimetricreagents can be used for detecting different metals. For example,pyrocatechol violet (PV) is a sensitive reagent for calorimetricdetermination of tin, iron, aluminum, chromium and yttrium. Othercalorimetric reagents sensitive to tin may include galleon, pyrogallolred, bromopyrogallol red, or phenylfluorone.

Pyrocatechol violet is a sulfone phthalein dye made from condensing twomoles of pyrocatechol with one mole of o-sulfobenzoic acid anhydride.Pyrocatechol violet has four hydroxyl groups, two sulphone oxygens and aquinone oxygen. Chemical structure of pyrocatechol violet is as follows

All of the functional groups are in conjugation with a delocalizedaromatic system that imparts color to the compound by absorbing light inthe characteristic visible wavelengths. Pyrocatechol violet has colorindicator properties and forms complexes of blue to blue-violet colorswith metal ions.

In an exemplary embodiment, a method of inspection or detection of ametallic material on a cleaned surface of a casted article by usingpyrocatechol violet is illustrated in a flow chart 80 of FIG. 4. Theinspection method involves a step 84 of applying hydrochloric acid onthe surface of the cleaned casted article. Hydrochloric acid convertsany tin, iron, aluminum, chromium or yttrium present on the surface, toa corresponding chloride. Further, the inspection method involves step86 of exposing the surface to pyrocatechol violet. In one embodiment, apink color (block 88) is observed due to formation of protonatedpyrocatechol violet. Protonation of the oxygen by the hydrochloric acidcauses shift in frequency of the absorption of visible light because ofan altered electronic structure. The pink color indicates (block 90)that there is no trace of the above metals on the surface as shown bystep 92, and hence a clean surface (block 94). Thus, formation of thepink color ensures that the surface is free of tin after the cleaningtreatment. In an alternate embodiment, a blue-violet color (block 96)shows formation of a metal-PV complex and indicates (block 98) thepresence of residual metals (block 100) at some portions of the surfaceand thus implies a contaminated surface (block 102).

In some embodiments, the detection of a specific metal present on thesurface can also be achieved by using a masking agent. The masking agentcan be applied to the surface before exposing the surface to thevisualization reagent. The masking agent allows the specific metal to beexposed to the visualization reagent and does not allow other metallicmaterials to be exposed to and react with the visualization reagent. Forexample, suitable masking agents that allow only tin to be inspected andmask other metals, are lactic acid and ascorbic acid. These acidsselectively reduce metals except tin. Other masking agent may beN-(2-hydroxyethyl) ethylenedinitrilotriacetic acid (HEDTA) orethylenediaminetetraacetic acid (EDTA), which selectively chelates othermetals.

Various techniques can be used to expose the surface to thevisualization reagent. A unit can be used to expose at least a portionof the casted article to a visualization reagent. Suitable units mayinclude, but are not limited to, a cotton swab, a block paper, a spray,a bath, a glass rod, a glass pipe, or a cloth.

Similarly, various techniques can be used to observe the response onexposing the visualization reagent to the surface. The response of thevisualization reagent may be observed with or without the use of avisualization aid. In one embodiment, the response may be observed bythe human eye. In another embodiment, the visualization aid may be anoptical system such as a light source, an optical detector, an opticaldifferentiator or a combination thereof. The optical differentiator mayinclude a filter, a prism, a spectrometer, an interferometer or thelike. For example, the emission of a fluorescent dye may be observedunder UV excitation.

In exemplary embodiments, some preferred techniques for using acalorimetric reagent are illustrated in FIG. 5A, 5B, and 5C. An article72 is the article recovered after step 24 of the cleaning method 14illustrated in FIG. 1. In one embodiment, the surface 74 of the article72 is free of any metallic material. Alternatively, the surface 74 mayinclude at least a portion containing the residual metallic material 76.In one embodiment, the unit may be a cotton-tipped swab 112 as shown inFIG. 5A. The swab 112 wet with the calorimetric reagent 110 is swipedover the surface 74 of the article 72 and a response is observed. Inanother embodiment, the unit may be a block paper 114 as shown in FIG.5B. The block paper 114 dipped into or impregnated with the calorimetricreagent 110 is pressed against the surface 74. Colors on the block papershows corresponding cleaned or contaminated portions of the surface 74.Yet in another embodiment, the calorimetric reagent 110 is sprayed overthe surface 74 by a sprayer 116 as shown in FIG. 5C. A color change canbe observed over the surface 74 at corresponding portions of the surface74.

EXAMPLES

The example that follows is merely illustrative, and should not beconstrued to be any sort of limitation on the scope of the claimedinvention.

Example 1

The following Example 1 illustrates a directional solidification processconducted utilizing a tin metal cooling bath. In this process, anarticle, for example, as turbine blade is first cast in a mold that ismade from Ni based superalloy. The superalloy article comprises 7.5weight percent cobalt, 7.0 weight percent chromium, 6.2 weight percentaluminum, 6.5 weight percent tantalum, 1.5 weight percent molybdenum,5.0 weight percent tungsten, 3.0 weight percent rhenium, trace amountsof hafnium, yttrium, boron and carbon, with the balance being nickel.The mold and casting are lowered into a bath of molten tin at a rate of0.5 cm/minute. The temperature of the molten tin is maintained at about300 degree Celsius, approximately 50 degree Celsius above the meltingtemperature of the pure tin. The thermal gradient measured in the castpart is 98 degree C./cm.

A deposit of metal contaminant having a thickness of about 500 micronsis located on the surface of the casted superalloy article. The metalcontaminant is primarily comprised of tin, due to the ingression ofliquid tin during the casting process. The ingression of liquid tinoccurs in a liquid-tin-cooled directional solidification process whenthe mold containing the casted superalloy article cracks while the moldis still immersed in the liquid tin bath. As the mold cools, it developscracks while still in the liquid tin bath. The liquid tin flows alongthe cracks in the mold and makes contact with the surfaces of the castedarticle inside the mold. The liquid tin can react with the interior ofthe mold while flowing through the mold cracks and with the superalloywhile in contact with the casted article surface. As a result, someelements in the casting mold and in the casted article may also bepresent in the metal contaminant.

The contaminated casted article is immersed in a bath of an aqueouscomposition comprising commercially available grades of approximately 71volume percent hydrofluosilicic acid (H₂SiF₆), 24 volume percentphosphoric acid (H₃PO₄), and 5 volume percent hydrochloric acid (HCl).The bath is maintained at a temperature of 80 degrees Celsius, while thearticle is immersed in the aqueous composition for 4 hours. The metalcontaminant is substantially removed by the aqueous acid compositionafter the approximately 1-hour immersion. The treated surface of thearticle is inspected to detect any residual tin present on the surface.The surface is first treated with concentrated hydrochloric acid. Thehydrochloric acid used is 10-volume percent acid in water. The tinpresent on the surface reacts with HCl to form tin chloride. A cottonswab dipped into a 10⁻³ M solution of PV is applied on the acidifiedsurface. The swab turns pink due to formation of protonated pyrocatecholviolet (PV).

Example 2

The contaminated casted article is immersed in a bath of an aqueouscomposition comprising commercially available grades of approximately 71volume percent hydrofluosilicic acid (H₂SiF₆), 24 volume percentphosphoric acid (H₃PO₄), and 5 volume percent hydrochloric acid (HCl).The bath is maintained at a temperature of 80 degrees Celsius, while thearticle is immersed in the aqueous composition for 4 hours. The metalcontaminant is substantially removed by the aqueous acid compositionafter the approximately 4-hour immersion. The treated surface of thearticle is inspected to detect any residual tin present on the surface.The surface is first treated with concentrated hydrochloric acid. Thehydrochloric acid used is 10-volume percent acid in water. Any tinpresent on the surface reacts with HCl to form tin chloride. A cottonswab dipped into a 10⁻³ M solution of PV is applied on the acidifiedsurface. The swab turns blue-violet due to formation of tin complex withpyrocatechol violet (PV).

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A method, comprising steps of: forming a casted article by a liquidmetal cooled directional solidification process; removing a metallicmaterial from a surface of the casted article; and inspecting for thepresence of the metallic material on the surface of the casted articleby exposing the surface to a visualization reagent.
 2. The method ofclaim 1, wherein removing the metallic material from the surface of thecasted article, comprises a chemical process.
 3. The method of claim 1,wherein the chemical process comprises contacting the metallic materialwith an aqueous composition which comprises an acid having the formulaH_(x)AF₆, wherein A is silicon, germanium, titanium, zirconium,aluminum, or gallium, and x is 1-6.
 4. The method of claim 3, wherein xis 1-3.
 5. The method of claim 3, wherein contacting comprises immersingthe casted article in a bath of the aqueous composition.
 6. The methodof claim 1, wherein the casted article comprises iron, cobalt, nickel,aluminum, chromium, titanium, or a combination thereof.
 7. The method ofclaim 1, wherein the casted article comprises a superalloy material. 8.The method of claim 1, wherein the casted article is an airfoil.
 9. Themethod of claim 1, wherein the metallic material comprises tin, iron,cobalt, nickel, aluminum, chromium, titanium or a combination thereof.10. The method of claim 1, wherein inspecting comprises the steps of:applying an extraction reagent to the surface of the casted article;exposing the surface to the visualization reagent, and observing aresponse of the visualization reagent.
 11. The method of claim 10,wherein the response of the visualization reagent results in one or moreof a calorimetric response, absorbance, scatter and fluorescence. 12.The method of claim 10, wherein the extraction reagent comprises an acidselected from the group consisting of hydrochloric acid, nitric acid,sulfuric acid, phosphoric acid, acetic acid, and a combination thereof.13. The method of claim 12, wherein acid is present at a 1 to 10% weightvolume.
 14. The method of claim 1, wherein the visualization reagentcomprises a fluoresent dye or an optically active material.
 15. Themethod of claim 1, wherein the visualization reagent comprises acalorimetric reagent.
 16. The method of claim 15, wherein thecolorimetic reagent comprises pyrocatechol violet.
 17. The method ofclaim 16, wherein pyrocatechol violet is present in the compositionrange of from about 10⁻⁵ M to about 10⁻³ M.
 18. The method of claim 16,wherein the color obtained by the calorimetric reagent is a dark bluecolor thereby indicating the presence of the metallic material on thesurface.
 19. The method of claim 16, wherein the color obtained by thecalorimetric reagent is a pink color thereby indicating the absence ofthe metallic material on the surface.
 20. A method, comprising: fillinga mold with a molten metal; immersing the mold into a cooling metallicliquid progressively, to cause a solidification interface to passthrough the molten metal; recovering a casted article from the mold;removing a metallic material from a surface of the casted article; andinspecting for the presence of the metallic material on the surface ofthe casted article by exposing the surface to a visualization reagent.21. A system, comprising: a heating furnace at a temperature above theliquidus temperature of a metal within a mold; a liquid cooling bathcomprising a cooling metallic liquid; a bath comprising an aqueouscomposition to remove a metallic material from a surface of a castedarticle; a unit to expose at least a portion of the casted article to avisualization reagent; and a visualization aid for observing a responseof the visualization reagent.
 22. The system of claim 21, wherein theunit is a cotton-tipped swab, a block paper, a sprayer, a bath, a glasspipe or a glass rod.
 23. The system of claim 21, wherein thevisualization aid is an optical system or human eye.
 24. The system ofclaim 23, wherein the optical system is a light source, an opticaldetector, an optical differentiator or a combination thereof.